The invention generally relates to the art of printed circuit board (PCB) manufacture and more particularly to the manufacture of a PCB having a stacked header component.
Many techniques are employed in the manufacture of PCBs in order to increase component density and reduce the area and/or size of the PCB. The exploded view of FIGS. 1A and 1B show the results of one such technique in which an integrated circuit (IC) semiconductor chip 20 is directly mounted onto a PCB 22 using a xe2x80x9cdirect chip attachxe2x80x9d process. In this process, a solder mask is placed on an etched or coined PCB and solder paste is screened onto the copper lands of the PCB. The PCB 22 is then stuffed with components, including IC chip 20. The IC chip 20 is not housed in a chip carrier or any other kind of package and thus is able to occupy a minimal footprint on one side the PCB 22 as compared to a fully packaged IC, e.g., one encased in a common dual in-line package (DIP). After stuffing, the PCB is heated by a heat radiation means to reflow the solder paste and electrically and mechanically connect the components to the PCB. The PCB is then washed to remove excess solder flux. Thereafter components such as the IC chip 20 are wire-bonded directly to the PCB. After wire-bonding and a potential testing phase, an encapsulant 24 is applied to the IC chip 20 and possibly other components in order to hermetically seal these components from the external environment.
In the illustrated embodiment, the PCB 22 includes a copper backplane 25 which provides a low profile means for dissipating heat. In circumstances where the IC chip 20 produces significant operating heat it is directly attached to the backplane 25 for efficient heat transfer. The encapsulant 24 protects the chip and wirebonds from the surrounding environment.
The component density of the PCB 22 is also increased by stacking a header 26 onto the PCB 22 after the encapsulant 24 is applied. The header 26 may carry on its underside 28 relatively large electronic components such as inductors 30 thereby eliminating the need to reserve a relatively large footprint on the PCB 22 for these bulky components. The header 26 includes a number of friction-fitted pins 32. A portion 32B of the pins extend from the underside 28 of header 26 for mounting it onto respective header pin lands 34 located on the PCB 22. Some of the pins 32 and correspondingly some of the lands 34 serve to electrically interconnect the inductors 30 to the circuitry of the PCB. A portion 32A of the pins 32 extend from a top-side of the header and may be used to mount the PCB/header assembly 22 and 26 to a host card or mothercard (not shown) in a larger system. In this case, some of the pins 32 and correspondingly some of the lands 34 may be electrically active and function as input/output interconnections between the PCB 22 and the host card. This feature also eliminates the need to dedicate a significant footprint of the PCB 22 for card edge connectors.
The header 26 is relatively large and may be sized as large as the PCB 22 itself as shown in FIGS. 1A and 1B, or may be somewhat smaller. Smaller headers may also be employed. As such, the header 26 must typically be mounted to the PCB 22 after the encapsulant 24 is applied. This creates certain thermal constraints in soldering the header pins 32 to the corresponding lands 34. The principal constraint is that solder located under the directly attached IC chip 20 should not be allowed to reflow once the encapsulant 24 is applied. This is because the chemical composition and temperature profile of solder paste changes after the first reflow. The solder underneath the chip 20 may contain a number of small voids which, when subsequently reflowed, may coalesce to produce a large void. A direct mount chip with a large solder void underneath it is unable to efficiently dissipate heat to the copper backplane 25 and thus will have a very short field life.
In the past, the pins 32 were hand-soldered to the PCB 22. This was a labour intensive and economically undesirable method of manufacture. The problem was exacerbated due to the thermally conductive copper backplane 25 which acted as an effective heat sink making it difficult to manually solder each pin.
Alternatively, a heat radiation and flux dispenser apparatus was employed to reflow solder (previously applied) on lands 34 in order to create a joint with the header pins 32. This apparatus was often unable to create successful joints. In cases where the lands 34 were very close to the site of the IC chip 20, e.g., less than 0.25 inches, the solder on lands 34 did not receive enough heat to reflow due to the aforementioned thermal constraint. If the heat radiation time was increased to reflow the solder on lands 34, solder would also reflow under the IC chip 20, creating unwanted voids and defective PCBs 22. The problem is exacerbated due to the rapid heat conduction properties of the thermal backplane 25 to which the IC chip 20 is directly attached.
Furthermore, in an effort to keep within the limits of the aforementioned thermal constraint, the apparatus was used to reflow only one side of the PCB 22 at a time in order to keep the temperature of the solder underneath the direct mount IC chip 20 below the solder reflow point. This uneven heating of the sides of the PCB caused header 26 to tilt and reduced the number of successfully soldered pins on the opposite side of the PCB in the following manner: One side of the PCB was heated first. Assuming that the voiding described above did not occur, the solder was reflowed on the first side and the header pins travelled downward due to gravity to touch the underlying copper-plated surface or land of the PCB on that side. However, the solder on lands on the second side of the PCB 22, being ball-like in shape, were still solid and high, causing the header 26 to tilt somewhat, with the first side down relative to the second side. The apparatus then advanced to reflow the solder on the second side of the PCB. However, the header pins 32 were high and would not travel down to meet the copper land of the PCB, since the header 26 is constructed from a solid plastic mould and the pins 32 are friction inserted into the plastic. This caused a great failure rate in the joints on the second side of the PCB.
Broadly speaking, the invention overcomes various problems of the prior art by employing a heat conduction, as opposed to heat radiation, approach to creating the header-PCB solder joint.
One aspect of the invention relates to a method for mounting a component having one or more pins onto a printed circuit board (PCB) having one or more respective lands for receiving the component pins. The method includes: (a) applying solder and flux, preferably in paste form, onto the lands; (b) bringing the pins in contact with the lands; (c) preheating the PCB to at least a flux-activation temperature; and (d) applying additional heat only to the pins in order for the pins to conduct sufficient heat to reflow the solder on the PCB lands.
The method may be advantageously applied to PCBs having pre-existing solder joints, such as an un-packaged IC chip directly mounted onto a copper backplane. In this case the PCB is heated in step (c) to a temperature approaching but not reaching the reflow temperature of the solder in the pre-existing joints, and in step (d) heat is applied so that the pins conduct only enough heat to locally reflow the solder on the lands without reflowing the solder in the pre-existing solder joints.
In the preferred embodiment the component is a header and its pins are exposed on top and bottom sides of the header. The top portions of the pins provide contact points for a heating element and the bottom portions of the pins provide a part for assembly onto the PCB.
The apparatus according to the preferred embodiment includes a nest for locating the header and the PCB in stacked alignment. A top and bottom heater apply heat to the PCB. The bottom heater receives the nest and provides a general heating of the PCB to at least a flux-activation temperature but less than the reflow temperature of the preexisting solder joints. The top heater includes a top heating block connected to an actuating mechanism such as a piston for bringing the heating block into contact with the exposed header pins for a time sufficient for the pins to conduct enough heat to locally reflow the solder on the lands.
In the preferred embodiment the top heating block is resiliently suspended from the actuating mechanism in order to reduce the impact between the heating block and the header pins. Thermally insulative material such as a ceramic shield is disposed between the heating block and the actuating mechanism in order to reduce heat transfer.
The heating block preferably features a satbilizer member resiliently suspended therefrom. The stabilizer member contacts and applies a light pressure onto the header in order to stabilize it prior to the heating block contacting the header pins. The stabilizer member also assists in stabilizing the header, whose recently formed solder joints are still substantially liquid, as the top heating block is retracted.
The heating block preferably features a plurality of teeth resiliently suspended therefrom, with each tooth being configured for separate contact with an individual header pin. This enables the heating block to comply with variations in the heights of the header pins.
The apparatus according to the preferred embodiment further includes a conveyor having a moving element for transporting the nest underneath the top heater. The bottom heater is embedded in the conveyor moving element. The nest is located on a carrier tray and the conveyor moving element is keyed to locate the carrier tray thereon. Lifters are also disposed proximate to a terminating end of the conveyor for raising the carrier tray off of the hot bottom heater in order to cool the former without operator intervention.